Secondary fibers recovered from waste paper are converted into a wide variety of recycled paper products. Waste paper materials often contain varying amounts of troublesome sticky or tacky contaminants including adhesives, binders, coatings, certain types of inks, and the like. These tacky contaminants, usually referred to as "stickies", accumulate and cause operational problems in the various pieces of process equipment used to screen and clean the pulp, form paper sheet, and handle paper sheet products. The presence of these tacky contaminants or stickies also can affect adversely the quality of the final paper products. The growing utilization of secondary fibers in many types of paper products has made it necessary for paper manufacturers to process lower grades of waste paper. Such lower grade furnish is more heterogeneous and typically contains more stickies than a preferred higher quality waste paper.
Economical manufacture of paper products using secondary fibers thus requires effective methods for treating secondary pulps to detackify and/or remove stickies (usually referred to as "detackification"). Known methods for removing or reducing the tackiness of stickies typically involve the addition of materials similar to those used to control pitch deposition in papermaking with virgin pulp. Such materials, defined herein as detackification agents, include surfactants, water-soluble polymers, and other ionic water-soluble compounds, as well as inorganic materials such as talc, diatomaceous earth, clays, and the like. These materials act to modify the surfaces of the dispersed particles of tacky compounds to remove the contact adhesive properties of the particles and thereby render them less troublesome and more easily removed. U.S. Pat. No. 4,710,267 discloses the use of surface-active tertiary amines or quaternary ammonium compounds for reducing discoloration and tackiness in waste paper pulps. U.S. Pat. No. 4,886,575 discloses the application of polyvinyl alcohol having various molecular weights for detackification of waste paper pulps. P. C. Miller, in a paper entitled "Chemical Treatment Programs for Stickies Control" in TAPPI Proceedings, 1988 Pulping Conference, pp. 345-348, reviews the stickies problem and describes a number of approaches to detackification by chemical addition to secondary pulps.
Oxygen treating or delignification of secondary pulp is known but generally has not been utilized to address the problem of stickies. U.S. Pat. No. 4,416,727 discloses a process for recovering and maintaining the brightness of fibers from wet-strength resin-coated waste paper furnish by pulping the furnish in an alkaline solution and contacting the pulp with oxygen to dissolve the polymeric wet-strength resins followed by recovery of the acceptable fibers in a washing step. An article by A. De Ruvo et al entitled "Upgrading of Pulp from Corrugated Containers by Oxygen Delignification" in Tappi Journal, June 1986, pp. 100-103 describes a process for upgrading recycled fibers by oxygen delignification to give the fiber a more virgin-like character by fiber swelling and softening resulting from lignin removal. The problem of stickies was identified but the removal of such stickies was not addressed. In an article entitled "Oxygen Bleaching of Secondary Fiber Grades" published in Tappi Journal, December 1988, pp. 168-174, L. D. Markham et al describe an oxygen bleaching process effective for removing most types of dirt and improving the brightness and bleachability of recycled pulps. The effect of the oxygen bleaching process on stickies was investigated and it was concluded that stickies were not well removed or dispersed in the oxygen stage.
The need for improved and more economical methods for detackification of pulps in secondary fiber recovery led to the present invention as disclosed and claimed below.